Well this is a capacitor.

OK, but what's inside of this?

Inside of this capacitor is the same thing

that's inside basically all capacitors.

Two pieces of conducting material

like metal, that are separated from each other.

These pieces of paper are put in here

to make sure that the two metal pieces don't touch.

But what would this be useful for?

Well, if you connect two pieces of metal to a battery,

those pieces of metal can store charge.

And that's what capacitors are useful for.

Capacitors store charge.

Once the battery is connected, negative charges

on the right side get attracted towards the positive terminal

of the battery.

And on the left side, negative charges

get repelled away from the negative terminal

of the battery.

As negative charges leave the piece of metal on the right,

it causes that piece of metal to become positively charged,

because now that piece of metal has

less negatives than it does positives.

And the piece of metal on the left

becomes negatively charged, because now it

has more negatives than it does positives.

It's important to note that both pieces of metal

are going to have the same magnitude of charge.

In other words, if the charge on the right piece of metal

is 6 coulombs, then the charge on the left piece of metal

has to be negative 6 coulombs.

Because for every 1 negative that

was removed from the right side, exactly 1 negative

was deposited on the left side.

Even if the two pieces of metal were different sizes

and shapes, they'd still have to store

equal and opposite amounts of charge.

Now I've only show negative charges moving,

because in reality it's the negatively charged electrons

that get to move freely throughout a metal,

or a piece of wire.

The positively charged protons are pretty much stuck in place,

and have to stay where they are.

This process of charge switching sides

won't continue to happen forever, though.

Negative charges on the right side

that are attracted toward the positive terminal

of the battery will start to also

get attracted toward the positively charged piece

of metal.

Eventually the negative charges will

get attracted to the positive piece of metal, just as much

as they're attracted toward the positive terminal

of the battery.

Once this happens, the process stops,

and the accumulated charge just sits there

on the pieces of metal.

You can even remove the battery, and the charges

will still just continue to sit there.

The negatives want to go back to the positives,

because opposites attract.

But there's no path for them to take to get there.

This also explains why the pieces of metal

have to be separated.

If the pieces of metal were touching during the charging

process, then no charges would ever get separated.

The negatives would just flow around in a loop

because you've completed the circuit.

That's why you want the paper in there,

to keep the two pieces of metal from touching.

So capacitors are devices used to store charge.

But not all capacitors will store

the same amount of charge.

One capacitor hooked up to a battery

might store a lot of charge.

But another capacitor hooked up to the same battery

might only store a little bit of charge.

The capacitance of a capacitor is the number

that tells you how good that capacitor is at storing charge.

A capacitor with a large capacitance

will store a lot of charge, and a capacitor

with a small capacitance will only store a little charge.

The actual definition of capacitance

is summarized by this formula.

Capacitance equals the charge stored on a capacitor, divided

by the voltage across that capacitor.

Even though technically the net charge on a capacitor

is 0, because it stores just as much positive

charge as it does negative charge.

The Q in this formula is referring

to the magnitude of charge on one side of the capacitor.

What the voltage is referring to in this formula

is the fact that when a capacitor stores charge,

it will create a voltage, or a difference

in electric potential, between the two pieces of metal.

Electric potential is high near positive charges,

and electric potential is low near negative charges.

So if you ever have positive charges sitting next

to, but not on top of, negative charges,

there's going to be a difference in electric potential

in that region, which we call a voltage.

It's useful to know if you let a battery fully charge up

a capacitor, then the voltage across that capacitor

will be the same as the voltage of the battery.

Looking at the formula for capacitance,

we can see that the units are going to be coulombs per volt.

A coulomb per volt is called a farad,

in honor of the English physicist Michael Faraday.

So if you allow a 9 volt battery to fully charge up

a 3 farad capacitor, the charge stored

is going to be 27 coulombs.

For another example, say that a 2 farad capacitor

stores a charge of 6 coulombs.

We could use this formula to solve for the voltage

across this capacitor, which in this case is 3 volts.

You might think that as more charge gets stored

on a capacitor, the capacitance must go up.

But the value of the capacitance stays the same.

Because as the charge increases, the voltage

across that capacitor increases, which

causes the ratio to stay the same.

The only way to change the capacitance of a capacitor

is to alter the physical characteristics

of that capacitor.

Like making the pieces of metal bigger,

or placing the pieces of metal further apart.

Just changing the charge or the voltage

is not going to change the ratio that

represents the capacitance.

[MUSIC PLAYING]